Threaded Mandrel for Unimat SL Chucks

[Paul Jones]

Thanks PJs. I published this simple machining project just to get the discussion going about using collet chucks in lathes. I made a comment the other day on one of the machining YouTube channels about where did the author buy the very simple 5C collet chuck for his lathe. One of the other viewers joined in the discussion and told me how to make one to fit in the MT5 taper on my 12" swing lathe by machining an unhardened MT5 to MT3 adapter. It was a great idea.

I am now considering making a 5C collet chuck for the 12" swing lathe by starting with an unhardened MT5 to MT3 adapter and boring the center larger and adding the 10 degree taper for 5C collets. I can make a simple threaded draw bar with a recessed shoulder to allow knocking the collet back out of the spindle. I think this will be an inexpensive way to add 5C collets to the lathe.

Back to you question, my import mini lathe (bought in 1993) has a MT3 taper in the spindle but it also uses the typical flange on the spindle for bolting the chucks and faceplate onto the spindle. You can take advantage of this way of attaching collet chucks to eliminate a lot of the TIR. Several years ago I bought the ER32 chuck from Little Machine Shop and the collet sets are a very good quality from Techniks (very low TIR). Most of the ER32 collet sets go up to 3/4" and I added the 25/3" and 13/16" sizes.

The chuck mounts to the spindle flange with three bolts and nuts. In practice the recess in the back of the collet chuck is machined to an exact fit over the spindle flange for a quick way (well not exactly "quick" when dealing with the loose bolts, lock washers and nuts) to mount the chuck. Instead I machined the recess in the back of the collet chuck to be about 0.003" larger in diameter. This allows adjusting the chuck to run true on the spindle using a gage pin in the collet, a dial indicator, snugged bolts and and soft hammer to make the adjustments. This works well for me when dedicating the lathe most of the time to collet work. Most of the work is for short parts so loosing an inch of working room is okay and could use steady rest. The longer work is performed on the 12" swing lathe with 37" between centers and its steady rest.

Regards,

Paul